Despite the wide diversity of available infrared light sources, there is no equivalent in the infrared of a light emitting device (LED), i.e. an intense light source having a limited infrared spectral linewidth, a good directivity of its emitted infrared light beam and which is easy to manufacture, so relatively cheap.
Instead, there are a great variety of thermal light sources which are used for low end applications. The limiting issues with these infrared light sources are mainly:                their infrared emission spectrum follows closely the one of a blackbody;        the emitted radiation is omnidirectional, such as a Lambertian angular distribution;        the light modulation is very limited and intensity modulation frequencies are typically smaller than some Hertz;        the conversion of the electrical energy in useful infrared radiation is poor;        the packaging of these light source requires often a vacuum or inert gas environment.        
The most common thermal source is a heated tungsten filament as the one found in light bulbs. These thermal sources are well known to have broadband emission in the visible and the infrared, which roughly follows the spectral dependence of a blackbody (FIG. 1). This can be generalized to all thermal sources wherein the optical energy is radiated thermal energy. The emission intensity at a given temperature and wavelength per unit wavelength and per unit angle in this case will be:I(T,λ)=∈(T,λ,θ)B(T,λ)
Where ∈(T,λ, θ) is the material emissivity and B(T,λ) is the blackbody radiation at the given temperature (T) and wavelength (λ), and θ is the emission angle. The blackbody radiation spectrum is a fundamental physical property that applies to all macroscopic systems and can show some variations in microscopic systems as explained in: H. P. Baltes, Infrared Physics 1976, vol. 16, pp. 1-8.
While the blackbody spectrum is difficult to change, for light source purposes for example, some attempts have been made to achieve this as explained in: A. Reiser and L. Sch{hacek over (a)}chter, Physical Review A 87, 033801 (2013). On the other hand, the emissivity can be changed in many ways. Through Kirchoff's law the emissivity and the absorptivity of a heated body are the same. Therefore the goal of engineering a thermal source emission can be solved by engineering the absorption of the thermal sources so that it achieves the required emissive needs.
There have been many attempts to overcome the issues of the infrared emission spectral width, its directionality and its modulation speed by using periodic structures to control the infrared light emission. These concepts can be divided into two groups, i.e. photonic crystals and sandwich structures such as metal-insulator stacks.
Photonic structures are used to inhibit the emission of certain wavelengths and the emission in certain directions. Variants based on both dielectric materials and metals have been demonstrated. The implementations using metals (woodpiles) are extremely difficult and costly to fabricate. Variants based on dielectrics, while showing a non-blackbody spectrum have strong off-band emission properties and it is difficult to obtain a well-defined emission spectrum centered on a chosen central infrared wavelength.
EP 1779418B1 describes a photonic crystal emitter in which infrared light is provided by the heating of a semiconductor material. The infrared light is transmitted by a metal layer. The limitation of the device of EP 1779418B1 is the limited modulation frequency that may be achieved and which is limited by the mass of the semiconductor material layer. Another limitation of the device is the limitation in the reduction of the spectral width that may be achieved with the device.
In the case of metal-Insulator sandwich structures, the most successful attempts to control the emission have been with absorbing dielectric layers that are sandwiched between two metal layers. Metal-Insulator sandwich structures have in general one continuous metal layer, acting as optical ground plate and heater, and one structured layer so as to provide path antennas, as described in: Xianliang Liu et al. “Taming the Blackbody with Infrared Metamaterials as Selective Emitted”, Phys Rev let. 107, 045901 (2011). Xianliung et al. describe in their paper a narrow band thermal emitter by using metallic crosses on top of an absorber on top of a continuous metal film. The absorption of the thin layer is enhanced significantly at certain resonant wavelengths leading to narrow band emission when the device is heated. The drawback of such a structure is again off-band emission. Also, the layered nature of the device is very sensitive to delamination under repeated temperature cycling of the device.